Field of the Invention
The present invention relates to an image processing apparatus, an image processing method and a storage medium.
Description of the Related Art
When printing a document including characters, graphics, photographs, images and the like, commonly, image formation using a screen having a low ruling is performed in photograph areas, and image formation using a screen having a high ruling is performed in character areas. When using a screen having a low ruling, the dots that are formed are very stable, and thus image formation with stable gradation is possible together with image formation with suppressed granularity (roughness). Therefore, it is preferred to use a screen having a low ruling for image formation in photograph areas. On the other hand, in the case of using a screen having a high ruling, the dots that are formed are fine, and thus detailed representation is possible. Therefore, it is preferred to use a screen having a high ruling for image formation in thin line areas and character areas.
Japanese Patent Laid-Open No. 2007-124086 discloses a technique for executing screen processing on each object, such as characters and photographs, using a screen selected from a plurality of screens whose screen angles are the same and whose screen rulings have an integer multiple relationship with each other. In Japanese Patent Laid-Open No. 2007-124086, even in the case where the screen ruling is switched for each object, favorable compression processing can be performed after screen processing.
It has become clear that, when comparing image formation using a screen having a low ruling and image formation using a screen having a high ruling, the consumption amount of a developing material (toner) during image formation is smaller when using a screen having a high ruling. This is because dots formed by a screen having a high ruling have a higher optical dot gain than dots formed by a screen having a low ruling, and even when the same amount of toner is used, can represent a higher density than a screen having a low ruling.
Therefore, in order to reduce running costs, it is desirable to apply a screen having a high ruling to not only character areas but also photograph areas. However, in the case of applying a screen having a high ruling to a low density portion (highlight portion) of an image, the dots that are formed are small and have low stability, and thus the gradation of the formed image deteriorates. Accordingly, it was difficult to realize gradation that has the same degree of stability as in the case of applying a screen having a low ruling while reducing the toner consumption amount by applying a screen having a high ruling.